Communications system

ABSTRACT

A communications system, including at least one base station ( 8 ) at the surface, at least one repeater station ( 6 ) below the surface, and at least one mobile station ( 4 ) below the surface, the stations ( 4, 6, 8 ) establishing a bidirectional communications path between the mobile station ( 4 ) and the base station ( 8 ).

The present invention relates to a communications system and, inparticular, to a bidirectional communications system which can operatebetween parties below, or a party on and a party below, the surface ofthe earth or of a body of water without reliance on any connectiveinfrastructure.

Communications through the earth, particularly in mines, has provedproblematic for a number of years and has generally required acommunications system which employs a connective infrastructure, such asthe use of leaky feeders, to aid communications signal propagation.Similar problems exist with electromagnetic communication betweenparties in an underwater environment or in any situation where thecommunicating parties are separated by a relatively lossy medium forelectromagnetic waves, such as the earth or water. The working range ofcommunications systems currently employed is limited, particularly in anemergency situation following an incident that removes any of theconnective infrastructure or air path between parties.

For instance, in an emergency situation in an underground mine, thedetermination of the location and establishment of the condition ofaffected persons, and the communication of proposed rescue strategies,is important. Communications systems are usually in place in undergroundmines and collieries to facilitate management and control of operationstogether with safety support through rapid response to calls forassistance. However, after an incident or disaster the infrastructure ofexisting systems cannot be relied upon to remain intact. Electromagneticwave propagation could be used for communication, but electromagneticpropagation through the earth is extremely range-limited due to theconductive characteristics of the medium. Radio communications directlythrough the earth that are relatively unaffected by strata conductivitycan only be performed using extra low frequencies (ELF) or very lowfrequencies (VLF), i.e. below about 30 kHz. Yet large antennas and largetransmit antenna energy requirements are necessary for useful range, andonly limited bandwidths are possible. A communications system haspreviously been developed using these frequencies to convey messagesfrom the surface of a mine to personnel underground. However, theestablishment of reliable communications in the reverse direction is amuch more difficult problem, due to: (i) the need to remain within safeantenna energy limits underground; (ii) the limited power sourcesavailable to be used in a portable transmitter worn by personnelunderground; (iii) the considerably larger amount of corruptive noise atthe surface, where a sensitive receiver is required to detect theextremely low signal reaching the surface through the lossy medium, and(iv) the small size of the antenna that is practical in mobileapplications.

In accordance with the present invention there is provided acommunications system, including:

at least one base station at the surface;

at least one repeater station below the surface; and

at least one mobile station below the surface;

said stations establishing a bidirectional communications path betweenthe mobile station and the base station.

Preferably the bidirectional communications path includes a first uplinkfrom the mobile station to the repeater station and a second uplink fromthe repeater station to the base station. Preferably the communicationspath also includes a downlink from the base station to the mobilestation and/or downlinks from the base station to the repeater stationand from the repeater station to the mobile station, respectively.

Advantageously at least a part of the communications path to and fromthe mobile station may be wireless. Preferably the downlink to and theuplink from the mobile station is wireless. Advantageously all of thecommunication links may be wireless. Preferably first carrierfrequencies between the mobile station and the repeater station andsecond carrier frequencies between the repeater station and the basestation are substantially different. For example, for communicationsthrough earth, the carrier frequency between the mobile station and therepeater station may be approximately 10 kHz and the carrier frequencybetween the repeater station and the base station may be approximately500 Hz.

The communications system preferably includes a plurality of therepeater station arranged in a cellular structure to cover respectivecommunication cells. Preferably the mobile station includes an antennawith a relatively reduced aperture compared to the apertures of therepeater station and the base station antennas.

The stations of the communications system preferably include linkmanagement means for monitoring characteristics of links between thestations, respectively, and for adapting communication parameters oflinks between the stations on the basis of said characteristics. Thecharacteristics may include link integrity and quality based on signalstrength and signal-to-noise ratio (SNR) data. The parameters mayinclude frequency, timeslot, modulation type and/or data rateallocation.

Advantageously, the system may further include a conductor magneticallycoupled to said mobile station to increase the communications path rangeof said mobile station.

Advantageously, said conductor may form a closed loop, and said loop mayadvantageously be made resonant at the frequency of operation to furtherincrease the communications path range of said mobile station.

The present invention also provides a communications system, including:

at least one base station;

at least one repeater station; and

at least one mobile station;

said stations establishing a bidirectional communications path betweenthe mobile station and the base station, and the communications pathfrom the mobile station to the repeater station is a magneticallycoupled wireless communications path.

The present invention also provides a communications system, including:

a receiver station;

a mobile station; and

conducting means magnetically coupled to said mobile station and saidreceiver station, said stations establishing a communications paththerebetween using said conducting means.

Preferred embodiments of the present invention are hereinafterdescribed, by way of example only, with reference to the accompanyingdrawings, wherein:

FIG. 1 is a block diagram of a communications system;

FIG. 2 is a block diagram of a preferred embodiment of a communicationssystem;

FIG. 3 is a block diagram of a second preferred embodiment of acommunications system;

FIG. 4 is a block diagram of a third preferred embodiment of acommunications system;

FIG. 5 is a block diagram of a fourth preferred embodiment of acommunications system;

FIG. 6 is a block diagram showing the time relationship for types ofcommunications frames transmitted between stations of the communicationssystem during a communications cycle;

FIG. 7 is a block diagram of the data frames used by the stations of thecommunications system;

FIG. 8 is a block diagram of a message part of the frames of thestations;

FIG. 9 is a schematic diagram of generation of the message part of FIG.8;

FIG. 10 is a block diagram of a first part of a receiver of thestations;

FIG. 11 is a block diagram of an output stage of a transmitter of thestations;

FIG. 12 is a block and flow diagram of modules of the receiver andtransmitter;

FIG. 13 is a block diagram of link and message management modules of thestations;

FIG. 14 is a diagram of a magnetically coupled conductive loop used toextend the range of a mobile station of the system; and

FIG. 15 is a diagram of a long conductor used to provide a magneticallycoupled loop.

A communications system for duplex communication with a person or partyunder a surface, as shown in FIG. 1, includes a combined control station(CS) 2 and base station (BS) 8, provided by a single unit, communicatingwith a mobile station (MS) 4 carried by a person under the surface. Theperson or party may be underground, such as in a mine, or underwater.The system establishes an uplink communications path (UL) and a downlinkcommunications path (DL) between the CS/BS and the MS. A repeaterstation (RS) 6 is used to establish the uplink path, which consists oftwo paths, ULA from the MS to the RS and ULB from the RS to CS/BS, asshown in FIG. 2. If desired, the communications system can also beconfigured so that the RS 6 is used in establishing the downlink path,which would then also have two components, DLA from the CS/BS to the RSand DLB from the RS to the MS, as shown in FIG. 3. The communicationssystem may include a single RS or a number of RSs to cover a widercommunications area and/or to provide redundant communication paths. TheRSs 6 are relatively fixed in location, whereas the MSs 4 are normallymobile and can be carried by a person. The system also ordinarilyincludes multiple MSs for respective individual users to provide amulti-user communications system. Depending on the communicationscoverage required, a plurality of base stations (BS) 8 may also beemployed.

FIG. 4 shows a general architecture for the communications system wherea plurality of base stations (BS₁ to BS_(n)) 8 are employed whichcommunicate with a central control station (CS) 2. A plurality ofrepeater stations (RS₁ to RS_(k)) and mobile stations (MS₁ to MS_(m))are also employed. The repeater stations (RS₁ to RS_(k)) are able tocommunicate with one another via dedicated multiple communication paths7 and the architecture of FIG. 4 establishes multiple downlink pathsDL₁₁–DL_(mm) between the base stations and the mobile stations. Multipleuplink paths are also established between the mobile stations and therepeater stations ULA₁₁–ULA_(mk) and between the repeater stations andthe base stations ULB₁₁–ULB_(kn). The uplink and downlink paths and thecommunication paths between the repeater stations are capable of beingwireless electromagnetic propagation paths which do not require anyphysical infrastructure, such as communication wires or leaky feeders.In practice, only a subset of these links would have an acceptably lowpath loss to form a practical communications system, for example due tointerference. Various subsets of the multiple propagation paths shown inFIG. 4 will provide a reliable communications infrastructure providedfunctional complimentary uplink and downlink paths are established. Therepeater stations 6 and base stations 8 are sited in such a manner as toprovide the required communication coverage and redundancy. For thepurposes of clarity, the following description of the preferredembodiments is provided in relation to a communications system having arepeater station 6 and a mobile station 4 under the earth, and inparticular in a mine. Electromagnetic propagation between the BS, MS andRS is therefore directly through the earth. The RSs 6 act as relativelyfixed repeaters and the MS 4 is normally used as a personal minerterminal. The control station CS 2 and base stations BS 8 may be placedon or above the surface or below the surface. Clearly additionalindirect downlink paths, as shown in FIG. 3 could also exist, but forthe purposes of clarity, these additional paths have been omitted fromFIG. 4.

The stations 4, 6 and 8 use antennas to minimise the link path loss,relying on the magnetic field linkage between antenna elements. Themagnetic field can be generated by causing a current to flow in a loopof wire of the antenna. This results in a transmitter whose strength maybe defined by its magnetic moment, which is the product of the currentand the effective area of the loop.

Even though the wavelength of the electromagnetic waves in theconductive medium is much less than when propagating in free space, forthe carrier frequencies used, the link ranges are within a small numberof wavelengths. This means that the communications system is notoperating in the ‘far field’. In other words the dominant energytransfer mechanism never gets to be a true electromagnetic waveconsisting of just quadrature electric and magnetic fields of equal peakenergy density. For the magnetic field generated by a loop this meansthere are no nulls in the beam pattern of the antenna. At any frequencythe link range is constrained by the skin depth, of the medium at thatfrequency. For a magnetic dipole antenna, in a lossless medium, the nearfield magnetic field varies as 1/r³, where r is the distance from themagnetic dipole. In a conductive medium, at a distance approximatelyequal to the skin depth, the additional exponential attenuation of themagnetic field strength with distance becomes significant. Link range isnormally considered to be limited to a distance where the exponentialdecrease is starting to predominate. The text book “Introduction toElectromagnetic Fields and Waves” by Corson & Lorrain, published by W.H. Freeman & Co, provides explanations on the field of a magnetic dipoleand the effect of skin depth.

Within the range that the system can operate satisfactorily, a receiveantenna can always be oriented to pick up a signal no matter what theorientation of the transmit loop. The easiest method of constructing alarge loop underground or on the surface is just to lay the wire on theground. With this arrangement the best coupling occurs when the bottomloop is laid out on the floor of the mine and located directly under thetop loop so the antennae axes are collinear. This relative positioningis also optimal if the loops are both rotated 90 degrees so that theloops are coplanar. This vertical coplanar loops configuration isgenerally not practical for large loops because of the difficulty inachieving sufficient loop area and hence, magnetic moment, in thetransmit antenna. However, when the transmit antenna is restricted insize (as for the portable antenna of the MS 4) coplanar orientation, ina horizontal plane, becomes the preferred one because of itsomni-directionality and the fact that it maximises range for thecommunications system. The actual coupling in any situation may becalculated from the full field equations for propagation in a lossymedium.

At the receiver, either absolute magnetic field, detected bySuperconducting Quantum Interference Devices (SQUIDs) or Hall Effectdevices, or rate of change of magnetic field, detected by loop antennas,can be utilised. The rate of change of magnetic field method, using theloop antenna as the detector, currently provides the best sensitivityfor mine applications. If the conductivity of the medium that the signalis propagating through, as well as the background magnetic noisespectrum, is known then it is possible to determine the transmitfrequency which maximises the receive signal SNR for a given propagationdistance and antenna aperture, as described below. For the propagationbetween fixed stations (BS and RS) and in general (depending ongeometry) between the base station BS and the mobile station MS thedistance the signal has to propagate over is known. This, together withthe known noise level at the receiver allows the optimum operatingfrequency and the minimum acceptable magnetic moment of the transmitterto be calculated. The fixed RS and BS stations are designed to operateat a frequency consistent with the calculated optimum frequency. At thebase station there is normally no problem in generating sufficientmagnetic moment.

Underground there are limitations on available power and on energystorage in the transmit antenna, for instance due to an intrinsic safetylimitation within coal mines. This, together with generally high noiselevels at the surface, is a limiting factor in the uplink. However, evenwith this limitation, the magnetic moment can generally be increased byincreasing the area of the loop for a RS, and given the distance betweenthe stations and the conductivity of the intervening material there isan optimum transmission frequency.

The situation for a MS is quite different. The size of the transmitteris physically constrained to be small and hence the maximum possiblemagnetic moment that can be generated is generally orders of magnitudesmaller than what is possible with a fixed installation. This in generalprecludes direct communications from a mobile underground transmitter tothe surface. Instead, advantageously the system sends all communicationsvia a relatively fixed RS. As the distance over which transmission canbe achieved is much smaller this means that the optimum transmitfrequency for the mobile station will be much higher than that of theother transmitters. For a known noise level at the receiver andtransmitter magnetic moment it is possible to optimise this frequencyand hence maximise the distance over which propagation is possible.

The communications system has a relatively low allowed message transferlatency and high required throughput in view of the low raw link datarate. The system architecture is driven by these requirements as well asthe need for robustness of communication in severely disruptiveconditions of a post-disaster situation.

A number of other factors were considered when designing thecommunications system and these are discussed below. In addition tounique design features, the system contains a number of features foundin modern digital communication systems, and in particular, wirelesssystems.

The major advantages of having a bidirectional communications systembetween a miner and the surface, apart from the ability to pass usermessages in either direction, relate to: (i) feedback error recovery;and (ii) automated verification of message delivery.

Feedback error recovery allows higher user message throughput to beachieved (at a given message error rate) than would otherwise bepossible on the link, while automated delivery verification increasesconfidence that the message has reached the desired destination andreduces the need for manual confirmation. The latter saving reduces theworkload of personnel and increases the speed with which actions may betaken. An extension of automatic message delivery verification iscontinuous link integrity monitoring, as described below, and, in thecase of a system architecture having a cellular structure, automaticlocalisation of individual miners at the cell level.

The underground transmitters of the RS 6 and MS 4 are often, for firesafety reasons, limited in peak external signal energy. It is alsodesirable that the peak internal energy level of the transmitters belimited to intrinsically safe values, if possible, to reducemanufacturing complexity (hence cost) and to simplify the approvalprocess. The restriction on peak signal energy means that linkthroughput will be maximised if modulation schemes producing low crestfactor signals are used. Low crest factor signals have an average energylevel which approaches the maximum possible for a given peak level, andthus maximise the bandwidth (and hence data rate) of the link for agiven received SNR. This consideration, together with the desire fortransmitter simplicity and power efficiency particularly for the mobileterminal 4, favours single carrier, constant amplitude modulationschemes such as m-PSK and m-FSK.

The carrier to noise power ratio at the receiver output for a givenrange and carrier frequency is proportional to the square of themagnetic moment of the transmitter. The peak energy stored in thetransmit antenna magnetic field is limited, as mentioned above, forsafety reasons. The achievable magnetic moment is, under theseconditions, an increasing function of the effective antenna aperture(e.g. loop area). A portable miner terminal uplink is likely (at a givendata rate) to be most severely limited in range (˜50 m at 25 bps QPSK),due to physical limits on transmit antenna size.

For any peak-energy-limited link in a lossy medium, such as wet rock,there is an optimum carrier frequency which maximises the carrier tonoise ratio at the receiver output. This frequency depends on the mediumconductivity, the range, and the normalised effective noise level (as afunction of frequency) at the receiver input. Assuming a typical case ofa −6 dB decrease in magnetic field spectral noise density for eachdoubling of the frequency, the optimum frequency can be determined as:Optimum frequency=k/(σμr ²) HzWhere:

-   -   k varies, depending on the direction of the receive loop        relative to the transmit loop, from 2.55 when the loop axes are        collinear to 4.75 when the loops axes are coplanar;    -   σ is the conductivity;    -   μ is the permeability; and    -   r is the range.

The magnetic moment of the transmit antenna, and noise levels at thereceiver provide constraints on the possible range for a given data rateand modulation scheme.

For a medium of conductivity 0.05 Ω⁻¹m⁻¹, corresponding to typicalsedimentary rock, the optimum frequency for a 300 m surface-to-minerdownlink is ˜400–500 Hz, while that for a 50 m miner uplink (with verymuch smaller transmit magnetic moment) is ˜10 kHz. In practice, thecarrier frequencies are chosen to be at or near odd multiples of 25 Hzin order to provide maximum immunity to harmonics of 50 Hz power mainsused in a number of countries, such as Australia. For countries using 60Hz power mains, the carrier frequencies would be chosen near oddmultiples of 30 Hz.

Using intrinsically safe transmit signal energy levels, the achievablerange from the miner terminal is limited due to the rapidly degradingSNR at the receiver. To maximise the range, a low data rate is used.With regard to message traffic requirements, every reasonable effort ismade to maximise the data rate and to minimise the non-productive linkoverhead. It was determined not to be practical to try to achieve auseable data rate over a direct link to the surface (˜300 m),particularly in view of the higher typical background interferencelevels at a surface receiving site. Thus, the system advantageouslyincludes an underground repeater 6 for the uplink. The data rate usedbetween the mobile terminal 4 and the repeater 6 would be at least 1 bitper second and generally higher, with multiple mobile terminalscommunicating with a single repeater terminal.

If it is practical for the repeater 6 to use a large area transmit loop(e.g. around a pillar), it is possible to achieve an intrinsically safelink to the surface, with an acceptable data rate of more than 10 bitsper second, using a carrier frequency of 400–500 Hz.

A practical implementation of the communications system therefore uses amultiple stage uplink via one or more repeaters 6, and transmits fromthe mobile terminal 4 at a carrier frequency of ˜10 kHz and to thesurface at 400–500 Hz from the repeaters 6. The downlink from thesurface can, as mentioned previously, go directly to the mobile terminalor via a repeater. In the former case, a low carrier frequency isrequired, while in the latter case, the miner-to-repeater link could usea carrier frequency up to and beyond 10 kHz.

If link channel bandwidth is at a premium, which is more likely to be aproblem at low carrier frequencies, simple QPSK (or QAM) modulation ispreferred, but if there is bandwidth to spare, it may be traded forincreased apparent effective baseband SNR and hence throughput. This isachieved by simply going to m-FSK modulation, but a more effectiveapproach is to use FEC coding to provide a higher raw bit rate for anymodulation scheme. If the channel bandwidth required exceeds ˜25 Hz, amodified m-FSK modulation scheme can be used to operate in such anenvironment.

At any range (r) the optimum frequency decreases as 1/r², as mentionedabove, and the received signal amplitude varies as 1/(r⁵) at thisoptimum frequency. This means that the range increase achievable bytechniques such as m-FSK and FEC bandwidth expansion is relativelysmall. It also means, however, that the vast majority ofminer-to-repeater links have a path loss which is substantially lessthan that at the range limit, particularly when the terminals 4 are in aposition well within the limit. Links within the range limit are capableof a significantly higher throughput, so an architecture which supportsadaptive variable rate links allows an overall message throughput whichis about 5 to 25 times higher than for links at the limit.

The preferred system configuration is point to multi-point two-way withdownlink broadcast capability and some level of message error controland delivery confirmation for each link direction. The message trafficis likely, under normal circumstances, to be predominantly downlink, andthe uplink may limit system throughput or response time, particularlywhen global or group broadcast messages requiring delivery confirmationare being sent from the surface. Under these conditions, the uplinktraffic will be dominated by delivery confirmation messages.

The uplink traffic will typically (in order of decreasing trafficvolume) comprise: (a) link integrity check data; (b) downlink messagereceipt acknowledgment data; and (c) uplink message data. Accordingly,the system handles link integrity checking and message receiptacknowledgment as efficiently as practical, even to the extent ofproviding different protocols for the three classes of traffic.

In order to provide acceptably low latency when delivering oracknowledging receipt of short length high priority user messages, thelink data block size is set to be relatively small compared to that usedin typical communication applications. This means that minimising theaverage absolute link overhead, per user message data block delivered,will assist in maximising system message throughput. The main componentsof the overhead are typically:

-   -   (a) transmitter and receiver hardware settling guard time.    -   (b) channel (media) access/grant information.    -   (c) receiver demodulator synchronisation information.    -   (d) data block boundary information.    -   (e) addressing information.    -   (f) message integrity check information.    -   (g) message delivery verification information.    -   (h) share of background link integrity monitoring information.

Minimising link overhead typically involves sharing overhead componentsamong as many data blocks as is practical and reducing the amount ofidle link time as much as possible, consistent with achieving acceptablelatency targets. These constraints, together with a need for“ruggedness” in the presence of noise bursts, favour “synchronous” fixedblock length protocols with adaptive traffic scheduling, as describedbelow. Such protocols also simplify the implementation of “batterysaving” operating modes.

A maximum range miner-to-repeater link ULA is likely to havesignificantly lower throughput than the repeater-to-surface link ULB. Itmay therefore be advantageous to have, within the service area of arepeater, many miner terminals active at the same time on differentcarrier frequencies. A difficulty with this approach is a potentialdynamic range problem with the receiver in the repeater, caused by anexcessively close miner terminal. There are, however, a number of waysaround this problem. These include remote control of the transmit powerlevel of the miner terminal and, more effectively, a miner terminal timeslot scheduling scheme which groups miner terminals 4 into severaldifferent time slots based on their received power level at therepeater(s) 6.

If simultaneous transmissions from several miner terminals are allowed,the repeater-to-surface link becomes the bottleneck, so the repeaterperforms an uplink data concentration function. In particular, it passeslink integrity and message delivery confirmation data from individualminer terminals to the surface in composite messages containinginformation from several terminals. This provides useful gains, becausemedia access and error recovery overheads can then be shared, and thereis no need to pass on non-productive poll responses.

As mentioned above, even with a thermal noise-limited link, FEC codingand detection increases the effective link SNR (and theoreticalthroughput) compared to the uncoded case. For noise that is burst-likeor impulsive, the potential benefits are substantially greater, so FECcoding is incorporated into the system, particularly when it is intendedto operate in an environment where impulsive interference (fromelectrical contactors, etc.) is present.

The efficiency of error detecting and error correcting codes drops offrapidly as the number of data bits “covered” by a particular check bitdecreases, so the coding gains achievable with the relatively short datablocks of the system will fall short of those obtained in more typicalcommunication systems. On the downlink, the problem may be reduced byapplying coding over an error control block containing data packets witha variety of destinations, providing the latency target can be met, andthe increased processing overhead at the miner terminal does notcompromise battery life. This scheme is not applied on the minerterminal-to-repeater link, because the data is coming from differentsources. Furthermore, coding gain is worst for the most common uplinktraffic, link integrity data, since this consists of only a few bits pertransmission.

Notwithstanding the lower coding gains achievable, it is still desirableto use FEC coding on the miner terminal uplink, particularly since burstor impulsive noise (for which the effective coding gain is higher) maybe present. The limited channel bandwidth available may, however,require a tradeoff between the allocation of bandwidth to FEC redundancyas opposed to concurrent transmissions from multiple terminals. If, forexample, a mean coding gain of 3 dB can be achieved with a rate 1/2code, the uplink capacity of an individual terminal can be doubled, butthe occupied bandwidth is quadrupled. The aggregate uplink capacity fora given available channel bandwidth is thus halved compared to theuncoded case. In this situation, coding may be favoured because if thereis sufficient channel bandwidth it requires half the transmit energy perdata bit and so increases battery endurance for the miner terminal 4.Information on code rates can be obtained from a text book such as“Digital Communications” by Bernard Sklar, published by Prentice-Hall.

If the aggregate channel bandwidth required to support theminer-to-repeater links of all miner terminals in an installation of thesystem exceeds the actual channel bandwidth available, a cellularchannel reuse scheme is introduced. This is practical since the smalluseable range of the miner-to-repeater link means that many repeaterswill be needed in a typical mine installation, and the rapid drop infield strength with range means that channels may be reused inrelatively nearby cells. A cellular channel reuse scheme introducessignificant link overheads associated with mobility management, so it isonly considered if other measures do not provide sufficient aggregatetraffic handling capacity for the system. In particular, variable ratesignalling schemes (and even use of higher modulation order) areemployed before applying channel reuse.

On the downlink, there is a choice as to the source(s) of the usermessage traffic and the various link overhead information elementsintended for a particular miner terminal 4. The simplest approach is topass all information directly from base station 8 to terminal 4, andnone via the repeater 6, but this may not allow an acceptable aggregatedownlink user message throughput. Ultimately, the simplest topologygiving acceptable throughput, latency and reliability is chosen. Thiscan be a hybrid, with system timing and user message traffic comingdirectly from the base station 8, while access control and linkintegrity verification traffic comes from the serving repeater(s) 6. Therepeater-to-terminal links would, in this case, operate at non-optimalfrequencies above those in demand for other purposes—that is, above 10kHz. This is practical because the repeater transmit antenna aperture(and thus magnetic moment) would typically be many times larger thanthat of the miner terminal, so the additional path loss at somewhathigher frequencies could be tolerated.

Regardless of the link topologies chosen, the system architecturesupports continuous or automatic fall-back redundancy at all levelsabove the individual miner terminal 4. Whether such capability isinvoked/provisioned is then a choice for the installer of the system. Inparticular, it would be desirable if redundancy at the repeater levelwere provided by ensuring that any active area of the mine was served bymore than one repeater. This would provide the greatest tolerance ofmechanical damage, and would enable the use of lower cost repeaters withno internal redundancy.

The system also executes “graceful degradation” (such as reverting tolower link symbol rate) in the presence of excessive noise, and theminer terminals revert automatically to a “beacon” mode if individuallink integrity is lost, as described below.

Since all repeaters share a relatively small available channel bandwidth(400–500 Hz carrier) for communication with the surface, this is likelyto limit the ultimate up and downlink traffic capacities of the system,unless an additional (higher capacity) route exists. This route could bevia a cable link between the base stations 8 and repeaters 6 which couldalso feed electrical power to them under normal working conditions. Allrepeaters would maintain radio link integrity monitoring with thesurface at all times, and under emergency conditions, those repeatersisolated from the cable feed would operate on standby battery and useradio for communication with the surface. This reduces traffic capacityby an amount dependent on the number of isolated repeaters, but it wouldnever be less than the entirely wireless system.

A simple two link system, as shown in FIG. 2, will typically requirerelatively small surface receiving antennas spaced at ˜300 m intervalsabove the active areas of the mine. If this is impractical, the numberof antennas may be reduced by employing the repeater-to-repeater links.These links are an additional drain on the channel bandwidth resources,but they can operate at the higher carrier frequencies (above 10 kHz)where there is more bandwidth available. Implementation of channel reuseon these links is also relatively simple and efficient, because they arestatic. The surface transmit antenna may be a single large loop, or 2–3concentric loops.

Although verification of downlink message delivery and confidence in thetwo-way integrity of the communication channel between the surface andthe miner terminal are very valuable, there are situations whencontinuous connectivity does not exist, and under these circumstancesthe system should still provide as much utility as possible. The minerterminal 4 can continue to receive downlink user messages, but at areduced data rate and with no delivery guarantee, and entry of uplinkmessages is allowed, in readiness for automatic transmission to thesurface as soon as the terminal can again communicate with a repeater.These capabilities support degraded repeater coverage under emergencyconditions or in those areas of the mine where full coverage is notconsidered to be cost effective.

An underground communications system which incorporates the abovefeatures is described below.

A system control entity broadcasts synchronous timing, timeslotallocation, and link frequency information etc from the CS 2 via the BSs8 for the communications system, as shown in FIG. 5. This isdisseminated to the repeater stations 6 and to the individual mobilestations 4. This information is varied as required in response to thechanging propagation environment and traffic load presented to thevarious links. The link 10 between the CS 2 and the BSs 8 may be a fixedelectrical or optical link, or a wireless link, and can be omittedaltogether when a CS 2 and BS 8 are combined. With regard to the messagecontent passed on the other links 12 to 18, the uplink 14 from the RS 6to the BS 8 passes repeater station link integrity/quality information,MS link integrity/quality information and MS uplink traffic, which maybe regenerated and aggregated traffic. The downlink 12 from the BS 8 tothe RS 6 passes system timing, access grant information relating to timeand frequency, and other control data. A downlink 16 to the MS 4, whichmay contain elements originating either from BSs or RSs, can also beused to provide system timing and access grant information detailingagain time and frequency and up and downlink information, together withbroadcast messages, directed messages (ie messages addressed to specificterminals) and other control data for the mobile station 4. The uplink18 from the mobile station 4 to the repeater station 6 passes dataconcerning: link integrity/quality information; broadcast messageacknowledgement; direct downlink message acknowledgement; direct uplinkmessages; uplink channel access requests; and beacon information.

The ruggedness and throughput/latency performance of the system istypically limited by the MS to RS uplink 18. Generally, more than one RSreceives an acceptable signal, having good SNR, from a particular MS andpasses traffic from that station to the BS(s), together with estimatesof the received signal strength and SNR. The control station 2 combinesthese redundant uplink streams to give a logical uplink of enhancedruggedness and reliability, which is not dependant on the availabilityof any individual RS or BS. It also uses the link integrity/qualityinformation from the MS, together with the signal strength and SNRforwarded from the various RS(s), to determine the actions necessary tooptimise communication with individual MS(s) and the aggregatethroughput of the system. This is achieved by varying the downlink anduplink data rate for individual MS(s) and/or handing the MS over toanother operating frequency and/or adjusting the timeslotallocation/duration, as described below. If the RS to BS uplinks arelimiting the throughput of the system, dynamic, traffic basedoptimisation can be performed on these links as well. A RS providesconcentration/editing of the traffic from individual MS(s), under thecontrol of the CS. Under these circumstances, the CS commands anindividual RS to suppress traffic from all but an individual dynamicallydetermined subset of the MS, chosen to ensure that no more redundantlinks are provided than are necessary to give adequate redundancy foreach MS. Furthermore, only changes in MS link integrity are signalled inorder to minimise overhead traffic. The architecture provides: linkintegrity monitoring; broadcast message delivery with confirmation;directed message delivery in both directions with confirmation; andlocalisation of the MS(s).

A typical system traffic cycle is shown in FIG. 6. The MS may provide amultiple of concurrent MS to RS frequency channels, and similarly the RSmay provide a multiple of concurrent RS to BS frequency channels, asshown in FIG. 6. The component fields of the BS frames 60, the MS statusframes 62, the MS message frames 64 and the RS frames 66 are shown inFIG. 7. The user/system message part 68 of the BS and RS frames and theMS message frame is shown in detail in FIG. 8. The user/system datasubpart 70 of the user/system message part 68 of FIG. 8 may be generatedfrom an original user message 72 by compressing the original message 72,and then segmenting it into segments 74 that are FEC encoded to producethe subparts 70.

Multiple access on the uplink is by time and/or frequency divisionmultiplexing, combined with access scheduling. Multiple access on thedownlink is by explicit station addressing and/or by time division.There may be transmissions from one or more mobile stations in timesequence, or concurrently (on separate frequencies) and thetransmissions from various types of entity may, in some embodiments,take place simultaneously or alternatively in a partially or totallynon-time overlapping manner. The BS, MS and RS phases may (dependant onthe channel frequencies used and equipment performance considerations)be distinct, as shown in FIG. 6, or time overlapped. If overlapped, theoverlap may be partial, so that an individual entity (MS, etc.) is notrequired to transmit and receive at the same time. Although the variousphases are shown as being of fixed duration, it is apparent that thedurations could be modified as a result of adaptation mechanisms.

An RS can be configured to handle a multiple of receiver channels, atdifferent carrier frequencies, received on respective antennas of theRS.

The base, mobile and repeater stations all execute the same basictransmitting and receiving operations, and the basic receiver andtransmitter components are the same, as described below, withconfiguration differences between the stations.

The receiver front-end of each station, as shown in FIG. 10, uses a loopantenna 76 to detect the signal. This loop can be air-cored, or be woundon a ferrite or other magnetically permeable core. In addition, it maybe tuned or untuned. The mobile station 4 uses a tuned ferrite-coredloop to maximise the received signal in a small volume. At the repeaterstation a single untuned air-cored loop is used with an effective areaof 50 m² turns. Being untuned, it is able to detect the 575 Hzsurface-to-repeater transmission as well as the 10,575 Hz mobile stationto repeater transmission. If tuned loops are used, then two separateantennas may be needed, but the system can be made smaller, and morerobust. The output of the loop is low-pass filtered and amplified by anamplifier 78 to a level suitable for an A/D converter 80. The subsequentoperations are executed in software, as described in detail below, butcan be performed in many different ways, and it is possible to do theseoperations with analog circuits. For example, the receiver may beimplemented with an analog frequency down-converter preceding the A/Dconverter 80 to reduce system clock rate. In the receivers of thesystem, the operations executed by respective software code componentsare comb filter 82, first bandpass filter 84, decimation by 16, secondbandpass filter 86, decimation by 5, and then I,Q quadrature mixing 88to produce a baseband signal. Following this is a signal detection andtiming extraction process. When the signal timing has been determined,the timing operation is used to control a demodulator (in the presentcase either BPSK or QPSK). Data is then sent to a detector for decoding.This then goes to a link protocol state machine, which also generatesoutput data and control for the transmitter modulator, as describedbelow with reference to FIG. 12.

The transmitter for the stations 4, 6 and 8, as shown in FIG. 11, passesthe baseband data for transmission to a digital modulator 90. Themodulator output is connected to a D/A converter 92 which drives a TXamplifier 94 (various types, such as class A, AB and C amplifiers can beused). In the surface and repeater stations the transmit antenna 96usually consists of a single turn of wire with about a 50 m diameter.Large diameter antennas can provide better coverage and range. For themobile station the transmitter antenna may be a coil wound on a ferriterod (usually tuned to minimise battery drain) or included in a wearabledevice, for instance in a vest or belt.

For each of the three stations 4, 6 and 8 the transmitter and receiverantenna may be the same physical structure, together with a switchdevice to switch the use of the antenna between the transmitteramplifier and the receiver.

In the transmitter, all operations executed before the D/A converter 92,are executed by software, and, as mentioned above, all operations afterthe A/D converter 80 of the receiver are executed by software. Thesebasic operations are the same for each of the three stations 4, 6 and 8,and are described below with reference to FIG. 12. The operations canalso be executed, as will be understood by those skilled in the art,wholly or partly by specific hardware circuits, such as applicationspecific integrated circuits (ASICs) using either digital and/or analogprocessing techniques.

Time synch and acquisition modules 20 handle system-wide timesynchronisation and coordination of transmit/receive activity on thevarious wireless links. Time synchronisation is established andmaintained with reference to a synchronous global timing cycle, followedby all stations of the communications system. This cycle provides a timereference for the start and end of all transmission frames as well asthe components of these flames and the individual link data symbols.Synchronisation of this global cycle is achieved by the periodictransmission of synchronisation headers from the CS/BS subsystem to allother elements. These headers are uniquely distinguishable (by carrierfrequency, modulation, coding, etc.) from all other informationtransmitted within the system, so as to facilitate recognition andtracking by the RS and MS stations. The synchronisation information isextracted by each of the slave stations with the aid of digital delaytracking loops. This provides substantially increased resistance tocorruption of system timing due to impulsive noise and otherinterference.

The modulator 22 and demodulator 24 use BPSK/QPSK modulation and avariant of differentially coherent detection. The common control andbroadcast information is transmitted at a fixed data rate whileindividual directed user and system messages are transmitted at a datarate determined by a link management entity, described below. The timingof all the transmission/reception relative to the system cycle is alsodetermined by the link management entity.

The BS transmissions do not, typically, have the same power restrictionsas MS and RS transmissions, so higher aggregate data rates, for a givenreceived SNR, can be achieved. The required data rate (per BS) toachieve adequate downlink traffic capacity may result in an RF bandwidthrequirement exceeding the frequency spacing of harmonics of the local ACpower frequency which form the dominant source of interference at thesefrequencies (below ˜2 KHz). In this case, a variant of m-FSK ormulti-carrier modulation which makes use of the RF bandwidth betweenseveral successive interfering harmonics, is used.

An FEC decoder 26 and encoder 28 use a forward error correction (FEC)block code, such as the Hamming code, together with data interleaving toimprove the error rate performance in the presence of impulsive noiseand other interference. Where possible, the block size for FEC ismaximised by aggregating related traffic. A cyclic redundancy checksum(CRC) for use in automatic retransmission request (ARQ) error controland message confirmation schemes is applied by the checksum generator 30to the relevant portions of the various message frames, and extracted bythe checksum validator 32. As with FEC, the block size is maximisedwhere possible for increased coding and detection efficiency. Wherepossible, individual traffic components are aggregated to give a largerblock size for FEC/CRC processing. In particular, BS frames can containuser/system parts directed to different RS/MS elements, and RS flamescontain traffic aggregated from several MS.

A Link Protocol State Machine 34 handles link access control, ARQprocessing and delivery confirmation. There are multiple link protocolsproviding overall system synchronisation/control, MS uplink accessmanagement, directed message ARQ/delivery confirmation and broadcastmessage ARQ/delivery confirmation. Broadcast message deliveryconfirmation information is contained within MS status flames whiledelivery confirmation of directed message frames is contained withinreverse-path (MS, RS or BS) message frames. This arrangement leads tooptimum message throughput and minimum delivery confirmation latency.Both message types rely on a “windowed” ARQ scheme, based on flamesequence numbering, which maximises throughput on the links, and thebroadcast message protocol is arranged to deliver messages as rapidly aspossible to the majority of MS elements.

A message segmentation module 36 and a message assembly module 38handle, respectively, message disassembly and reassembly. Fixed-lengthsystem/user data subparts are used in frames sent over the variouslinks, so variable length user messages are split into sections whichform the subparts, as shown in FIG. 9. If there is only one messagecurrently queued for a particular route, the last subpart is terminatedwith a unique end of message (EOM)/PAD sequence. If another message isalready queued, a subpart may contain both the end of the first messageand the start of the second message, with a unique (short) EOM/SOM(start of message) sequence separating them. Source coding is used withuser messages to provide data compression and so increase the usermessage throughput of the system. This consists of a variant of Huffmancoding which encodes, to form variable length symbols, commonly usedwords and phrases in addition to text characters. Such a coding schemeallows efficient representation of EOM and SOM sequences. For example, aterminating “?” or “!” in the message text may be encoded as part of aspecialised (and short) EOM symbol. In general, a different encodingdictionary is used for downlink and uplink, because user message trafficis likely to have substantially different content/statistics in the twodirections.

A user interface 40 provides significantly different functionality andpresentation services for the various stations CS, BS, RS and MS. The MSuses a robust single- or multi-line LCD display and short-form keypad.The CS uses a multi-window, multithreaded graphical user interface withfull keyboard, but with the ability to generate common messages andcommands via a “point and click” GUI. It also incorporates message timestamping and logging facilities together with an “undelivered message”reminder and network integrity display. The BS and RS user interfacesare mainly used for setup and maintenance purposes, but also incorporatethe normal user message functions to assist installation. They arepreferably implemented using a removable keyboard/display unit.

In general the various user interfaces include the following functions:

-   -   (i) Message entry/editing.    -   (ii) Message prioritisation.    -   (iii) Message transmission progress display.    -   (iv) Received message display (with progressive update).    -   (v) Message history (and logging for CS).    -   (vi) Link (MS, RS, BS) or network (CS) status display.    -   (vii) System control (CS).    -   (viii) Diagnostic/maintenance interface.

The system executes traffic scheduling to assist in maximising the usermessage throughput and minimising the transport latency. In the MS andRS this is achieved by user message prioritisation, queuing and uplinkaccess request initiation 44. In the CS, message prioritisation andqueuing is also performed, but, in addition as shown in FIG. 13, amessage scheduling entity 56, working with a master link managemententity 52, provides adaptive system optimisation by adjusting frequency,timeslot, modulation type and data rate allocation on the various datalinks. Individual transmit entities (described as a particular source,and a particular traffic type) are delineated by timeslot and/or carrierfrequency. A slave link management entity 54 in each communicationelement (MS, RS, BS) is responsible for local control of the timeslots,carrier frequencies, modulation types, and modulation rate, in responseto information signalled to it from the master link management entity 52in the CS. The slave entities also generate/interpret linkintegrity/quality data for the master entity. The entities 52 and 54 areexecuted by the link protocol state machine 34.

The functionality of the entities 52, 54, 56 varies from minimal (in thecase of the MS) to relatively complex (in the case of CS). The messagescheduling entity 56 is responsible for message transmissionprioritisation and scheduling as well as network traffic bandwidthmanagement while the link management entity 52, 54 is responsible forthe support of adaptive frequency, timeslot and data rate allocation onthe various data links. The two entities 52 and 56 work together in theCS to adapt the system configuration in such a way as to maximise systemuser message throughput and to minimise transport latency whenthroughput is not the limiting factor.

The master message scheduling entity 56:

-   -   (i) determines the relative proportions of traffic bandwidth to        be allocated to broadcast messages, directed messages, network        status monitoring and network configuration control.    -   (ii) controls retransmission (ARQ) scheduling on the downlink        and the uplink.    -   (iii) provides traffic bandwidth allocation commands to the link        management entity 52.    -   (iv) implements various system operating modes, such as normal        communication, emergency (manually directed) communication and        beacon search.    -   (v) provides message queue status (transmit progress monitoring)        display information to the CS user interface.    -   (vi) provides “undelivered message” display information to the        CS user interface.    -   (vii) provides user message I/O in conjunction with the user        interface.

The link management entity 52, 54:

-   -   (i) configures the RS repeater 6 behaviour so as to maximise        aggregate message throughput while providing adequate MS to RS        link redundancy.    -   (ii) issues frequency, timeslot and data rate allocation        commands to MS, RS and BS elements.    -   (iii) facilitates communication with a rapidly moving remote MS        (for example a vehicle or cage borne MS) by ensuring it has        potential access to all RS repeaters.    -   (iv) provides MS location and status display information to the        CS user interface.    -   (v) provides network performance/integrity display data to the        CS user interface.

The stations 2, 4, 6 and 8 of the system are also configured to executethe following functions:

-   -   (a) Downlink-only mode. This is a mode of operation (protocol)        which provides one-way downlink communication from CS/BS to MS,        without the need for RS uplink repeater stations. This mode,        which can coexist with the normal bidirectional communication        mode, would be of value after an event which has destroyed the        RS repeater infrastructure in a region of a mine or in those        cases where it is decided that it is not cost effective to        provide full RS coverage of all regions of the underground mine.    -   (b) MS beacon mode. This is a free running mode of the MS which        does not require external synchronisation (from CS/BS or RS),        but which provides an interference resistant one-way beacon        signal which maximises range for a given peak transmit field        stored (inductive) energy. The signal is designed to facilitate        (by use of a unique pseudo-random transmit time and/or        modulation pattern) unique identification of a particular MS in        the presence of other MS beacon signals, while minimising power        consumption for a given mean time to uniquely identify each        miner terminal. The mode is potentially useful in emergency        situations for locating trapped individuals when, for some        reason, the full system, particularly the CS/BS, is not        functional.    -   (c) MS and RS transmit control. The ability to remotely suspend        and restore (by command from the CS) transmissions from selected        MS and/or RS elements is useful in situations where power        consumption must be minimised (e.g. to extend battery endurance)        or background co-channel and adjacent-channel interference must        be controlled. These situations may occur during search and        rescue operations for trapped individuals.    -   (d) Frequency/phase locked transmission. An extension of the        synchronous system timing regime is the frequency/phase locking        of MS and/or RS transmissions, via the BS frame transmissions,        to a precision reference in the CS/BS subsystem. This is        particularly useful in the case of the MS, because it allows        synchronous detection and long term averaging of MS        transmissions when searching for or communicating with an MS at        ranges exceeding the normal maximum range of the MS-to-RS link.        It avoids the need for a high accuracy (less than 1        part-per-million drift) frequency reference in each MS and RS.

A limitation of the communications system described above is the limitedmagnetic moment that can be generated by a compact portable transmitterof the mobile station. The magnetic moment is limited to about 1 Am² ifintrinsic safety is to be maintained. In underground mines this maylimit operating the distance between the receiver and transmitter toless than about 70 m. Most mines cover a number of square kilometresunderground and thus the coverage of each receiver to mobiletransmitters in the mine may only be a fraction of 1% of the total mine.

To increase the operating range one approach is to introduce a number ofspatially separated receivers all connected to the one repeater station.This will increase the total area covered by each repeater station. Forexample, nine receivers with just overlapping coverage increase theeffective range by a factor of 3. Say from 70 m radius to a 210 mradius.

Each remote unit can be very simple. At the minimum it simply consistsof a passive receive loop connected back to the repeater station. Suchan arrangement is inherently intrinsically safe. For higher sensitivitythe remote unit would incorporate a preamplifier/buffer. In this case,intrinsic safety requirements are still easily met.

Another approach is to use magnetically coupled but electricallyisolated loops 100 of electrical conductor, within magnetic couplingrange of the mobile station 4 and repeater station 6, as shown in FIG.14. Any conductor that forms a closed loop 100 will act to make thetotal change in magnetic flux passing through the loop zero. It doesthis by way of an electric current that is induced in the loop by themagnetic field 102 of the transmitter. This current produces a magneticfield 104 that opposes the field generated by the transmitter within theloop (Lenz's Law). Outside the loop the two fields add. Thus, in theregion outside the loop, the magnetic field produced by the current inthe loop augments the magnetic field generated by the transmitter. Thebasic principle being used is that a time varying magnetic field inducesan emf and hence causes a current in a closed loop of conductor. Thesecurrents produce their own magnetic field and it is this extra magneticfield which will be used to improve the range of the system.

Within a mine it is usual to leave pillars which support the roof. Thenature of the magnetic field within the pillar 106 is of no importancebecause it is inaccessible. Thus placing a conducting loop around apillar will increase the magnetic field in the area outside the pillar.Because the current range of the system is comparable with the size ofpillars in a mine it is expected that the performance improvement willonly occur when the portable transmitter is in the same gallery orcut-through as the conducting loop. In this configuration it is expectedthat the magnetic field at the far side of the pillar is largely due tothe current in the conducting loop. In effect the loop is acting as a“magnetic relay” in the magnetic link between the transmitter and thereceiver transferring the magnetic field that would have been generatedwithin the pillar to a region uniformly distributed around the pillar.Wrapping the next pillar along with a conducting loop can extend thisconcept. The edges of the two loops would share the same mine galleryand we have a situation where the conductors in the two loops runparallel for some distance. This would provide coupling from one loop tothe next and so further extend the range. The loop can be made fromsteel hawser and hence be made very robust.

The condition for the total change of magnetic field within the loopbeing zero occurs when the impedance around the loop is dominated by itsinductance. According to the Faraday induction law the magnitude of thevoltage (V) induced around the loop is equal the rate of change in fluxthrough the loop. This voltage then induces a current (I) in the closedloop where. I=V/Z where Z=R+jωL with R is the resistance, L theinductance of the loop. With R small the inductance dominates Z and thetotal change of flux through the closed loop is forced to zero due tothe field generated by I.

If instead of shorting the loop a capacitor C 108 is included withimpedance equal in magnitude to that due to the inductance of the loop(that is 1/jωC=jωL) to join the ends of the loop, the total impedancearound to loop is now just R. The current and hence augmenting magneticfield is now increased by the Q of what is now a resonant circuit.Q=ωL/RThus tuning the loop with a capacitor 108 can increase the augmentationand further increase the range.

The conducting loop may partly or completely use existinginfrastructure. In a mine there are usually many long conductors, suchas water pipes and power cables. A magnetic transmitter situated next tosuch a conductor will induce an emf distribution along it. Depending onthe earthing of the conductor, this emf distribution can cause currentswhich travel for a considerable distance along the conductor. Thus itwill be advantageous to use receivers and or transmitters that areclosely coupled to such a conductor.

The most advantageous situation is a long conductor 110, as shown inFIG. 15, grounded at both ends. This in effect forms a loop with thereturn path through the ground. In this case, the induced current isuniform along the length of the conductor and communications can beestablished with any mobile station in or near the tunnel in which theconductor is located. It will also be advantageous to install extraconductors to take advantage of this effect. Also instead of loops, longconductors can be used to achieve the same magnetic coupling, induce acurrent in the conductor and generate a field which increases the rangeof the mobile station.

Many modifications will be apparent to those skilled in the art withoutdeparting from the scope of the present invention as herein describedwith reference to the accompanying drawings. For example, the system mayprovide for the exchange of user messages between mobile stations. Alsoat least one mobile station may be used for the control and monitoringof infrastructure rather than communications to a miner.

1. A communications system, including: at least one base station above aplanetary surface; at least one repeater station below the planetarysurface; and at least one mobile station below the planetary surface;said base, repeater and mobile stations establishing a bidirectionalwireless communications path through said planetary surface between themobile station and the base station; and said communications path fromthe mobile station to the repeater station being a magnetically coupledwireless communications path, and having first carrier frequenciesbetween the mobile station and the repeater station and second carrierfrequencies between the repeater station and the base station that areless than 30 kHz.
 2. The communications system as claimed in claim 1,wherein said at least one base station is at the surface.
 3. Thecommunications system as claimed in claim 1, wherein the bidirectionalcommunications path includes a first uplink from the mobile station tothe repeater station and a second uplink from the repeater station tothe base station.
 4. The communications system as claimed in claim 3,wherein the communications path includes a downlink from the basestation to the mobile station.
 5. The communications system as claimedin claim 3, wherein the communications path includes downlinks from thebase station to the repeater station and from the repeater station tothe mobile station, respectively.
 6. The communications system asclaimed in claim 1, wherein said communications path between said basestation and said mobile station is a magnetically coupled path, having acarrier frequency less than 30 kHz.
 7. The communications system asclaimed in claim 1, wherein the first carrier frequencies aresubstantially higher than the second carrier frequencies.
 8. Thecommunications system as claimed in claim 1, wherein the antennas of thestations include conductive loops, and the antennas of the base stationand the repeater station have substantially collinear axes, and theantennas of the repeater station and the mobile station aresubstantially coplanar.
 9. The communications system as claimed in claim8, wherein a carrier frequency between stations is determined byk/(σμr²) Hz, where: k varies, depending on the direction of a receiveloop relative to a transmit loop of said antennas, from about 2.55 whenloop axes are collinear to about 4.75 when loop axes are coplanar; σ isthe conductivity of the transmission medium; μ is the permeability ofthe transmission medium; and r is the range of the communications path.10. The communications system as claimed in claim 1 or 9, wherein forcommunications through earth, a carrier frequency between the mobilestation and the repeater station is approximately 10 kHz and a carrierfrequency between the repeater station and the base station isapproximately 500 Hz.
 11. The communications system as claimed in claim8, wherein the mobile station includes an antenna with a relativelyreduced aperture compared to the apertures of the repeater station andthe base station antennas.
 12. The communications system as claimed inclaim 1 including a plurality of the repeater station arranged in acellular structure to cover respective communication cells.
 13. Thecommunications system as claimed in claim 1, wherein said stationsinclude link management means for monitoring characteristics of linksbetween the stations, respectively, and for adapting communicationparameters of links between the stations on the basis of saidcharacteristics.
 14. The communications system as claimed in claim 13,wherein said characteristics include link integrity and quality based onsignal strength and signal-to-noise ratio (SNR) data.
 15. Thecommunications system as claimed in claim 14, wherein said parametersinclude frequency, timeslot, modulation type and/or data rateallocation.
 16. The communications system as claimed in claim 15,wherein said modulation type includes one of QPSK, QAM and m-FSK. 17.The communications system as claimed in claim 16, wherein saidmodulation type allows feedback error recovery by including FECencoding.
 18. The communications system as claimed in claim 15, whereinthe data rate between said mobile stations and said repeater station isat least one bit/s and the data rate between said repeater station andsaid base station is greater than 10 bit/s.
 19. The communicationssystem as claimed in claim 13, wherein said at least one base stationincludes a control station for said stations.
 20. The communicationssystem as claimed in claim 19, wherein the control station includes amaster of said link management means for controlling said adapting ofsaid communication parameters.
 21. The communications system as claimedin claim 20, wherein said control station includes message schedulingmeans for determining traffic bandwidth allocation and generatingtraffic bandwidth allocation commands for said master link managementmeans.
 22. The communications system as claimed in claim 19, whereinsaid repeater station concentrates and edits traffic on saidcommunications path, under the control of said control station.
 23. Thecommunications system as claimed in claim 13, wherein messages for auser of said mobile station are segmented and encoded to providefeedback error recovery.
 24. The communications system as claimed inclaim 13, wherein said link management means and said communicationspath are adapted to execute link integrity monitoring.
 25. Thecommunications system as claimed in claim 1, wherein said linkmanagement means and said communications path are adapted to executebroadcast message delivery with confirmation.
 26. The communicationssystem as claimed in claim 13, wherein said link management means andsaid communications path are adapted to execute directed messagedelivery in both directions of said path with confirmation.
 27. Thecommunications system as claimed in claim 13, wherein said linkmanagement means and said communications path are adapted to enabledetermination of the location of said mobile station.
 28. Thecommunications system as claimed in claim 1, wherein said at least onemobile station has a beacon mode during which said mobile stationgenerates a unique beacon signal detectable in the absence of saidcommunications path.
 29. The communications system as claimed in claim1, having a downlink mode establishing a unidirectional link from saidbase station to said mobile station without reliance on said repeaterstation.
 30. The communications system as claimed in claim 1, wherein atleast part of said communications path is through a partially conductivemedium.
 31. The communications system as claimed in claim 30, whereinsaid partially conductive medium comprises rock.
 32. The communicationssystem as claimed in claim 30, wherein said partially conductive mediumcomprises water.
 33. The communications system as claimed in claim 12,wherein said plurality of repeater station execute a frequency re-usescheme for carrier frequencies of different and concurrentcommunications traffic of said communications path.
 34. A communicationssystem, including: at least one base station_above a planetary surface;at least one repeater station below the planetary surface; and at leastone mobile station below the planetary surface; said base, repeater andmobile stations being adapted to establish a bidirectionalcommunications path through said planetary surface between the mobilestation and the base station, and the communications path from themobile station to the repeater station being a magnetically coupledwireless communications path; wherein at least part of saidbidirectional communications path is through a partially conductivemedium, and carrier frequencies of said bidirectional communicationspath being less than 30 kHz.
 35. The communications system as claimed inclaim 34, wherein said partially conductive medium comprises rock. 36.The communications system as claimed in claim 34, wherein said partiallyconductive medium comprises water.
 37. A communications system,including: at least one base station_above a planetary surface; at leastone repeater station below the planetary surface; and at least onemobile station below the planetary surface; said base, repeater andmobile stations being adapted to establish, without reliance on anyconnective infrastructure, a bidirectional communications path throughsaid planetary surface between the mobile station and the base station,and the communications path from the mobile station to the repeaterstation being a magnetically coupled wireless communications path, andcarrier frequencies of said bidirectional communications path being lessthan 30 kHz.
 38. The communications system as claimed in claim 37,wherein at least part of said communications path is through a partiallyconductive medium.
 39. The communications system as claimed in claim 38,wherein said partially conductive medium comprises rock.
 40. Thecommunications system as claimed in claim 38, wherein said partiallyconductive medium comprises water.